1. Field of the Invention
The present invention relates to a zoom lens, a variable magnification group, a camera unit having a photographic lens using such a zoom lens, a digital camera, a video camera, and a portable information terminal unit as an information equipment having a digital image photographing function.
2. Prior Art
The digital camera market is being extraordinarily expanded. Photographing of digital image is extended not only to a digital camera but also to a portable information terminal unit of portable telephone, and demands of users for digital camera are multi-laterally branched. Among them, high image quality and miniaturization are always desired by the users and are given much weight among the various desired items. For this reason, high performance and miniaturization are also required to be compatible in a zoom lens used as a photographic lens.
In view of miniaturization, first of all, it is require to reduce the total length of lens (the distance from the most object side lens surface to an image surface). Furthermore, in a camera unit of so called “collapsible mount style” that is contemplated for compactness at the time of receiving a lens, it is important to reducing the thickness of each group, which moves at the time of zooming, in the direction of optical axis in order to reduce the dimension at the time of reception.
Although various types are considered for the zoom lens for digital camera, there is a type which is appropriate for miniaturization, which comprises a first group with a negative focal length, a second group with a positive focal length, a third group with a positive focal length in turn from the object side, and an iris provided in the object side of the second group to move integral with the second group, wherein the second group monotonously moves from the object side to the image side at the time of variation of magnification from the short focus end to the long focus end, and the first group moves to correct the variation in position of an image surface accompanying the variation of magnification.
For example, although Japanese Patent Publication No. Hei. 10(1998)-039214 discloses all of the basic construction before everything else among those of which proposed the zoom lenses of the above type, it is not always sufficient in the aspect of miniaturization. Although a zoom lens improved over the above type and advanced in miniaturization is disclosed in Japanese Patent Publication No. Hei, 11(1999)-287953, the miniaturization is not sufficient and also aberration correction cannot to be said as sufficient because only one aspherical surface is provided in the second group. Although Japanese Patent Publication No. 2000-089110 discloses a zoom lens which is contemplated for good correction of aberration using the two aspherical surfaces in the second group as its embodiment 3, it is not always advantageous in miniaturization because the thickness of second group is large.
Also, because the examples of prior art which are disclosed in these publications are not appropriate in the construction of each group, in particular the second group, sufficient aberration correction do not take place and do not have a performance which is able to cope with an image sensing device of 2,000,000 to 4,000,000 pixels.